It is known that sphingosine-1-phosphate (hereinafter referred to as “S1P”) is a bioactive lipid that regulates cell proliferation, survival and migration and plays an important role in angiogenesis and lymphocyte migration (see, for example, Non-Patent Document 1). The concentration of S1P in the circulating blood and tissues is tightly regulated by a plurality of enzymes, and particularly, S1P lyase (hereinafter referred to as “SPL”) which is an enzyme irreversibly cleaving S1P. SPL is considered to be an essential factor for forming an S1P concentration gradient between blood and lymphoid tissues necessary for circulation of lymphocytes. In fact, in an analysis using SPL knockout mice, not only a dramatic increase in the concentration of S1P in lymphoid tissues, but also a significant decrease in the number of lymphocytes in the circulating blood is observed (see, for example, Non-Patent Document 2). Further, it is reported that 2-acetyl-4-tetrahydroxybutylimidazole (hereinafter referred to as “THI”) which has the activity of decreasing the number of lymphocytes in the peripheral blood inhibits the formation of an S1P concentration gradient by decreasing the SPL activity in lymphoid tissues in vivo (see, for example, Non-Patent Document 3). From these findings, it is expected that an SPL inhibitor may be used as a novel immunosuppressive agent based on the activity of decreasing the number of lymphocytes in the circulating blood for the avoidance of transplant rejection or various autoimmune diseases.
As for screening for an SPL inhibitor, several assay methods using SPL enzymatic activity in a cell-free system as an index have been reported (see, for example, Non-Patent Documents 4 to 6). However, an inhibitor specific to SPL has not been reported so far. Also for the above-described THI, although THI exhibits SPL inhibitory activity in vivo, the in vitro activity thereof has not been confirmed. On the other hand, it has been known that the accumulation of intracellular S1P is induced by inhibiting SPL. However, there have been no examples of cell-based screening using this accumulation as an index. Several methods for measuring the amount of intracellular S1P are already known (see, for example, Non-Patent Documents 7 to 10). However, all of the methods require a complicated lipid extraction operation, and their low sample processing capability is considered to be a problem. Further, as for the above-described THI, its activity has not also been confirmed in an experimental system using cultured cells. It is considered that this is because the existing experimental conditions are not optimized, and therefore, its activity cannot be detected.